With the recent popularization of computers, inkjet printers have been widely used for printing letters on paper, film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of ejecting a liquid droplet by applying a pressure using a piezoelectric element, a system of ejecting a liquid droplet by generating a bubble in an ink by heat, a system of using an ultrasonic wave, and a system of ejecting a liquid droplet by drawing with electrostatic force. As these inks for inkjet recording, there are used an aqueous ink, an oily ink and a solid (fusion-type) ink. Of these inks, the aqueous ink becomes mainstream in terms of production, handleability, odor, safety and the like.
A coloring agent used in such an ink for inkjet recording is required to have high solubility in a solvent, the possibility of high-density recording, good color hue, excellent fastness to light, heat, air, ozone, water and chemicals, good fixing property and less blurring on an image-receiving material, excellent storability as an ink, no toxicity, high purity and availability at a low cost. However, it is very difficult to seek a coloring agent satisfying these requirements in a high level.
Various dyes and pigments for inkjet recording have been already proposed and actually used. However, a coloring agent satisfying all requirements is not yet discovered at present. It is difficult to allow conventionally well-known dyes and pigments having a Color Index (C.I.) number to achieve compatibility of color hue and fastness required for the ink for inkjet recording.
As black dyes for inkjet recording, there have hitherto been widely known a food black dye, a naphthol-based direct azo dye, an acid azo dye and the like.
Typical examples of the food black dyes include C.I. Food Black 1 and C.I. Food Black 2, and techniques for using these in black inks for inkjet recording are described in JP-A-2-36276, JP-A-2-233782, JP-A-2-233783 and the like.
Further, the acid azo dyes include C.I. Acid Black 2, 31, 52, 140 and 187, and techniques for using these in black inks for inkjet recording are described in JP-A-60-108481, JP-A-2-36277, patent JP-A-2-36278 and the like. Furthermore, the direct azo dyes include C.I. Direct Black 9, 17, 38, 51, 60, 102, 107, 122, 142, 154 and 168, and techniques for using these in black inks for inkjet recording are described in JP-A-56-139568, JP-A-61-285275, JP-A-3-106974 and the like.
The present inventors have gone ahead with studies of inks for inkjet recording in which dyes are used. However, the present inventors have found out that aqueous black inks have the problem of low image durability.
As means for improving the image durability, particularly ozone resistance, of dyes for these black inks, the present inventors have proposed to increase the oxidation potential of the dyes in JP-A-2003-306623 and JP-A-2004-115620.
Further, JP-A-2004-115620 proposes that an interaction between dye molecules is strengthened to allow an existing state of a dye on an inkjet image receiving paper to resemble that of a pigment, thereby improving the fastness of the dye. However, when the interaction between dye molecules is strengthened as described above, the viscosity of a dye solution increases in some cases.
Such an increase in viscosity provides the larger influence with an increase in dye concentration in the aqueous solution.
By the way, when an ink such as an ink for inkjet recording is produced, a dye powder is generally used. However, considering handleability, solubility and the like, it is advantageous to produce the ink from a concentrated ink, that is to say, an ink stock solution.
However, it has been revealed that, for example, when a dye having a high oxidation potential is used in the ink stock solution for inkjet recording used as a raw material for the ink for inkjet recording, the viscosity becomes extremely high, which causes a large problem with regard to handleability at the time when the ink is supplied or filtered.
Such an increase in viscosity of the ink stock solution is a common problem to yellow dyes, magenta dyes and cyan dyes as well as black dyes.